1. Field of the Invention
The invention relates to a process for producing tantalum(V) nitride pigments which are notable for a higher a* color value in the CIE Lab system than predisclosed tantalum(V) nitride pigments. The invention furthermore relates to-tantalum(V) pigments having an a* color value according to CIE Lab of equal to/greater than +45; such pigments are obtainable by the process according to the invention. A further subject of the application relates to the use of the pigments.
2. Description of Related Art
There is a need to replace known pigments which may liberate, during use or during waste disposal, for instance in refuse combustion, toxic or ecologically harmful heavy metals by those pigments which are safe. Such a safe pigment is tantalum(V) nitride whose color is reddish-orange to red. The tantalum(V) nitrides obtainable by known processes have too low an a* color value in the CIE Lab system (DIN 5033, Part 3) and, in addition, appear slightly grayish or brownish. In accordance with H. Moureau and C. H. Hamplet (J. Amer. Chem. Soc. 59, 33-40 (1937)), tantalum(V) nitride can be produced by ammonolysis of TaCl.sub.5 ; the reaction times are very long and, in addition, a coloristically attractive product is not obtained. According to G. Brauer and J. R. Weidlein (Angew. Chem. 77, 218-219 (1965)), tantalum(V) oxide can be nitrided at 860 to 920.degree. C. with ammonia. In copying the above-mentioned process, the inventors of the present application found that, despite prolonging the reaction time, no further color displacement from reddish brown to red takes place so that this product is also coloristically less attractive.
According to the process described in DE-OS 42 34 938, tantalum(V) nitride can be obtained by nitriding a tantalum(V) oxide hydrate at 750 to 950.degree. C. with a shorter reaction time. The a* color values obtainable by this process on the basis of the test method described in said document was around +40. The a* color value was increased only by 2-3 units by thermal treatment of the Ta.sub.3 N.sub.5 at 300.degree. C. From a coloristic point of view, on the other hand, as high as possible an a* color value is desired, in particular a value above 45 and, particularly preferably, above 50.